Most of them, sure. But killing them with any kind of regularity would have a number of knock-on effects that would severely decrease many people’s quality of life.

If your friend has a brain tumor, you don’t point a gun to their head and shoot it out. You find brain surgeons and have them remove it under controlled conditions. Supposing you can’t find a brain surgeon, it would still be better to learn brain surgery yourself and do a careful and thorough job than it would be to just shoot your friend in the head and hope for the best.